Technical Field of the Invention
The present invention relates to a high speed high resolution heterodyne interferometric method and system, which can be used for measuring a high speed target using a laser source with low beat frequency.
Technical Background
The heterodyne interferometer has been widely used in many precision machines and calibration services due to its large measuring range, high signal-to-noise ratio and high measurement precision. Nowadays, high resolution, high speed and high accuracy measurements are required for many applications in order to produce small features with competitive production throughput. This initiates an improvement of the performance of the heterodyne interferometer to satisfy industrial demands.
The periodic nonlinearity caused by frequency and polarization mixing effects limits the accuracy and resolution of a heterodyne interferometer. A lot of research has been done to reduce the periodic errors, but the traditional heterodyne interferometer can not eliminate the frequency and polarization mixing effects, which make it difficult to realized high resolution and high precision displacement measurement.
T. L. Schmitz and J. F. Beckwith proposed a heterodyne interferometer with a acousto-optic modulator as a beam splitter. The measurement beam and the reference beam are spatially separated, which can reduce the frequency and polarization mixing effects. Therefore, the periodic nonlinearity is reduced; the precision and resolution are also improved. But its specific and complicated configuration limits the typically possible applications for measuring displacement.
Ki-Nam Joo, Jonathan D. Ellis, et al. proposed a simple heterodyne interferometer, which has spatially separated reference beam and measurement beam. This interferometer can eliminate the frequency and polarization mixing effects. The periodic nonlinearity is also eliminated and the precision is improved. Its configuration is simple which make it applicable to precision industrial engineering. But the measurement speed is limited by the beat frequency of laser source.
In conclusion, the existing methods and apparatuses described above isn't suitable for high resolution, high speed and high accuracy measurements.